(1) Field of the invention
The present invention relates generally to a structure of a system for automatically controlling a vehicle speed to a desired cruise speed.
(2) Background of the art
Various types of systems for automatically controlling a vehicle speed to a desired cruise speed without depression of an accelerator pedal by a vehicle driver have been proposed.
In one of the above-described previously proposed systems for automatically controlling the vehicle speed to the desired cruise speed, when a vehicle driver operates an accelerator pedal to accelerate the vehicle and the vehicle speed reaches a speed at which the vehicle driver desires to cruise, the vehicle driver pushes a set/coast switch. At this time, the vehicle speed when the set switch is pushed is stored in a control circuit as a set cruise speed. Thereafter, the system automatically controls an opening angle of a throttle valve installed in a vehicular engine or of a carburetor in response to a command derived from the control circuit. When the driver desires the set speed to increase, an acceleration/resume switch is continually activated (turned on) and the control circuit issues a command to increase the opening angle of the throttle valve so that the vehicle speed is increased. When the acceleration switch is released, the instantaneous vehicle speed is set as a new set vehicle speed. When the driver desires to decrease the vehicle speed, the set/coast switch is continued to be turned on so that an engine braking occurs and vehicle speed is decreased. Thereafter, when the set/coast switch is released, the instantaneous vehicle speed is set as a new set cruise speed. When a cancel switch is operated, the cruise speed control is interrupted (refer to Japanese Patent Application First Publication (unexamined) No. Showa 58-81838 published on May 17, 1983.)
An input circuit of the previously proposed system for automatically controlling the vehicle speed to a desired cruise speed will be described below.
The set/coast switch and acceleration/resume switch are connected to an input control circuit via signal lines, respectively.
The input control circuit constitutes a part of the control circuit. Each signal line is connected with a resistor and a zener diode for protecting the input control circuit from an overload voltage.
The cancel switch is connected to each signal line via a diode. The input control circuit, i.e., control circuit is constituted by a microcomputer or logic IC.
When either the set/coast switch or acceleration/resume switch is operated (turned on), a signal voltage, divided by resistance values of the resistors, is applied to a first input terminal of the input control circuit.
The input control circuit receives the signal voltage and identifies the signal indicating a command to set the vehicle speed, a command to decrease the set vehicle speed, or a command to increase the vehicle speed. The control circuit then executes a predetermined control operation according to the command.
If the cancel switch is operated, the signal voltage is applied via both signal lines to both of the first and a second input terminals. The control circuit determines that the signal voltage applied thereto is a signal indicating a command to cancel the control operation and interrupts the control operation of the vehicle speed.
However, since both signal lines are used to transmit the three switch signals to the input control circuit in the previously proposed system for automatically controlling the vehicle speed to the desired cruise speed, one signal line is used to transmit the two levels of signal voltage; low (L) level when one of the operation switches is turned off and; high (H) level when one of the operation switches is turned on, so that the operation of either of the set/coast or acceleration/resume switch is identified.
If both of the signal lines transmit high levels of the signal voltages, the control circuit determines that the cancel switch has been operated.
The H level generated when the cancel switch is operated and that generated when either of the set/coast or acceleration/resume switches are operated have no significant difference in level from each other. Therefore, if one of the signal lines should be broken or one of the zener diodes connected between the respective signal lines and ground fails, proper identification of switch operations cannot be made.